Browne is also likely to go to the two US states of Washington and Colorado, which both voted last November to legalise the recreational use of marijuana.

Browne will visit Sweden, whose zero-tolerance take on drug policy and abstinence-driven approach to drug treatment have been advocated by Tory champions of a more restrictive approach. He is also likely to go to Japan, which also operates a hardline zero-tolerance policy.

The study – ordered by the home secretary, Theresa May, in response to the Commons home affairs select committee's call for a royal commission to report on how to reform Britain's 40-year-old drug laws – is likely to be completed by the end of the year.

May ruled out decriminalisation, saying there was no need for a fundamental rethink of Britain's approach to drugs. But she did say there was a need to learn from new evidence of what works in other countries and that any debate of alternative approaches should be based on clear evidence and analysis.

Browne starts the international study on Mondayin Portugal, where a policy of "depenalisation" – removing criminal penalties for personal possession of all illicit drugs but not making them legal – was introduced in 2001.

This move was matched by a sharp increase in the availability of drug treatment and harm reduction measures which have led to a fall in HIV infection rates and a decline in drug-related deaths. The Home Office team will visit the European monitoring centre for drugs and drug addiction, based in the capital Lisbon.

The Lib Dems have long been committed to a major review of Britain's drug laws, including considering decriminalisation.

Browne said he was keeping an open mind on the lessons that could be learned. "I'm proud of the UK's drug policy and there are strong signs our approach is working. Illegal drug use is at its lowest level since records began and far more people are leaving treatment free from dependency than ever before," he said.

"But I'm not complacent. The UK cannot deal with this issue in isolation – my counterparts around the world are grappling with the same challenge of addressing the misery drugs inflict on individuals and communities and the considerable damage done globally by drug traffickers. I'm keeping an open mind about what lessons we can learn."

Britain has adopted a system of temporary banning orders as each new drug has appeared, while the official body of drug experts, the Advisory Council on the Misuse of Drugs, assesses whether they should be permanently prohibited.

Drug policy around the world

Portugal

Abolished criminal penalties for the personal possession of small quantities of illegal drugs in 2001 up to 1g of heroin and 25g of herbal cannabis. Police issue users with a notice to attend a "dissuasion commission" within 72 hours. Addicts are required to attend treatment or face a range of possible sanctions. Prevention campaigns highlighting the harmful effects of cannabis and heroin are also given a high priority. Commons home affairs committee said the moves had clearly reduced public concern about drug use and was supported by all political parties and the police.

US: Washington State and Colorado

Eighteen states and the District of Columbia allow the use of medical marijuana on prescription. Last November Washington State and Colorado voted to extend that to the legalisation of recreational use of cannabis.

In Colorado, adults aged over 21 are to be allowed to buy and possess up to an ounce (28g) of cannabis and grow six plants in a private, secure area. It also intends to license cultivation facilities and retail stores and introduce a tax on wholesale sales of marijuana, with the first $40m (£25m) used to build public schools. In Washington State the authorities intend to levy a 25% tax on marijuana sales and limit the number of retail outlets. Licences are to be issued by the state liquor control board.

The two state marijuana laws are in direct conflict with federal law and President Obama's drug policy head, Gil Kerlikowske, warned that the federal administration had no intention of honouring them.

Sweden

Has the toughest zero-tolerance approach to illicit drug use in western Europe. It has been widely praised by Conservative politicians and by advocates of the "war on drugs" for its low levels of illicit drug use, especially cannabis.

However, problematic drug use, including injecting amphetamines and heroin, is very high as a proportion of overall drug use. Tight restrictions on the use of methadone as a substitute for heroin has led to mortality rates among heroin addicts in Stockholm that are twice as high as other European cities. There are very limited harm reduction measures in Sweden, although needle exchange services have recently become legalised.

Both the use and possession of illicit drugs are illegal and minor use can attract a prison sentence of up to six months but usually leads to a fine. Sweden is unusual in making drug use itself a criminal offence in 1988 "in order to signal a powerful repudiation by the community of all dealings with drugs". Drug testing is compulsory for those suspected of having consumed drugs.

Denmark

Copenhagen is filled at weekends with Swedes taking advantage of the fact that cannabis is easier to buy there and alcohol is cheaper. Denmark's capital is planning a three-year experiment to place the criminal drugs trade under direct municipal control with sales restricted to Danish citizens. That may yet be blocked by the national government.

The Danes are the latest to join Switzerland and Spain in creating drug consumption rooms where chronic users can legally inject themselves with class-A drugs. Needle exchange programmes have operated in Denmark since 1986, and clean needles and syringes are now dispensed through machines in public sites. Methadone has been available since 1970.